cmd/compile: simplify zerorange on arm64

cmd/compile: simplify zerorange on arm64

Get rid of large zeroing cases. We only use this code
for small things now.

Change-Id: Iba0a98785c5b4b72cf031763edb69ff741ca41af
Reviewed-on: https://go-review.googlesource.com/c/go/+/678936
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <mark@golang.org>